Surface acoustic wave devices known as SAW devices have many uses in the UHF and VHF frequency ranges. SAW devices have been especially useful as impedance elements, resonators, and bandpass filters in these frequency ranges. Typical saw devices have a substrate with at least a surface layer of piezoelectric material and surface acoustic wave transducers in interdigitated form disposed on the piezoelectric surface. The transducers convert an electric signal to surface acoustic waves propagating on the piezoelectric surface.
SAW devices are compact, lightweight, robust, and, because they are a planar technology, are economical to manufacture. They can be mass-produced using the same techniques developed so successfully for the production of silicon integrated circuits. A wide variety of analogue signal processing functions can be achieved with SAW devices. Among other applications, they are currently used in pulse compression radar systems as receiver bandpass filters, or as resonators for stabilizing oscillators in numerous applications. They have replaced many of the coils, capacitors, and metal cavities of conventional radio frequency systems, removing the need for hand alignment and dramatically improving the reliability and performance. They have simultaneously resulted in significant reductions in both size and cost.
However, several problems are associated with the prior art surface acoustic wave transducers. One of the problems occurs because the transducer electrodes cause internal reflections which distort the transducer output and the shape of the input conductance. Another problem occurs when the transducer is used in filter applications. Triple transit distortion is caused by regeneration reflections between the transducers.
In order to eliminate triple transit distortion, three-phase and single-phase devices are used to cause a greater amount of radiation in one direction in the crystal than in the reverse direction and thus form unidirectional transducers. In one configuration proposed for a single-phase unidirectional transducer (SPUDT), a device such as that disclosed in U.S. Pat. No. 4,353,046 is constructed in which the acoustic reflections are used to cancel the regenerative reflections and unidirectional behavior results. These transducers are simple to fabricate and tune and thereby overcome many disadvantages of the multiphase devices. However, in early devices the finger and gap widths in a single-phase unidirectional transducer (SPUDT) were of split-finger construction and were one-eighth (1/8) of the operating acoustic wave length, thus limiting the frequency range of the device by photolithographic constraints to a maximum frequency of operation when compared to the simplest form of SAW transducer using quarter wavelength electrodes. As further explained in U.S. Pat. No. 4,353,046, the reason the split-finger SAW device becomes unidirectional is that alternate fingers of the split-finger electrodes are loaded with extra material. However, in experiments to date, insufficient distributed reflectivity has been achieved with this structure to produce useful directivity levels. The source of the problem was found to be that the reflectivity of .lambda./8 electrodes is generally very small. The reflectivity was also found to increase only very slightly with metal thickness. The reason for this unexpected behavior was finally identified as energy storage at the electrode edges.
With .lambda./4 electrodes, the reflectivity is unaffected by energy storage as the contributions from the front and back edges cancel. However, at other electrode widths they can significantly affect the value of the reflectivity. Unfortunately, experimental measurements show that for electrode widths of less than .lambda./4, the energy-storage reflections are generally of opposite phase to those resulting from the impedance discontinuities. The result is a substantial reduction in the electrode reflectivity for electrode widths less than .lambda./4. As a result of this phenomenon, all further development of this split-finger SPUDT configuration was discontinued.
Later, another two-level SPUDT configuration was proposed in U.S. Pat. No. 4,902,925, incorporated herein by reference in its entirety. This structure, commonly known as the "Hopscotch", employed a group type sampling with all electrode widths .lambda./4. Like the original SPUDT configuration, the first level of the "Hopscotch" transducer by virtue of the electrode groupings has no net internal reflections. Unidirectionality is only achieved by the addition of a second level metalization with this structure. However, since the electrode widths are .lambda./4, rather than .lambda./8 as in the original split-finger structure, greater internal reflectivity levels could be achieved. Unfortunately, as a result of the sparse group type sampling in the structure, the effective coupling is substantially reduced. The latter severely limits the maximum bandwidth or minimum insertion loss achievable with this transducer. In addition, this structure has significant group responses not far below the passband.
Independent from the "Hopscotch" transducer, another concept for a SPUDT was proposed which relies on unique crystal orientations as set forth in commonly assigned U.S. Pat. No. 4,910,839, incorporated herein by reference in its entirety. On these unique crystal orientations, a simple two-electrode-per-wavelength transducer exhibits unidirectional characteristics. Unfortunately, on these natural SPUDT (NSPUDT) orientations, the sense of directionality is determined by material properties of the crystal substrate and overlay material rather than by the transducer configuration as with other approaches. Reversing the sense of the directivity of the transducer thus turns out to be extremely difficult. To date, a low-loss filter comprising two NSPUDTs has not yet been built because of the difficulties in getting the two transducers to communicate with each other.
One solution to the NSPUDT dilemma would be to implement a low-loss filter with an NSPUDT and a group-type SPUDT (GSPUDT), such as the "Hopscotch" transducer. The filter function could then be implemented essentially by the NSPUDT, and the GSPUDT could simply be designed to have a sufficiently wide pass band and low loss. The undesired out-of-band group responses of the GSPUDT could be eliminated from the overall filter response by the NSPUDT. Unfortunately, the "Hopscotch" GSPUDT configuration cannot be made unidirectional on an NSPUDT orientation. This is a consequence of the sampling in the structure. Thus, an alternative GSPUDT configuration would be required for this application.
The present invention relates to GSPUDT structures with 3/8.lambda. and 5/8.lambda. sampling. Reflectionless or unidirectional transducers and broad-band notch elements are all implementable with these new configurations. The novel class of GSPUDTs disclosed herein is similar to the conventional SPUDT (CSPUDT) and the "Hopscotch" transducers in that single-level versions are reflectionless. Unidirectional characteristics are obtainable only from the two-level structure. However, in many cases they have significant advantages over either of the previous structures. These advantages are (1) they can be made unidirectional on either CSPUDT or NSPUDT crystal orientations; (2) they can be made unidirectional in either the forward or reverse directions by a simple change to the second level metalization; (3) they have substantially greater coupling, in some cases, than prior art transducers; and (4) they have greater minimum geometry, in some cases, than some prior art transducers. Thus, this type of transducer can be advantageously used in combination with an NSPUDT transducer to implement low-loss filters, resonators or the like.
The novel SAW transducers disclosed herein have a pattern of interdigitated electrodes on a piezoelectric substrate. The electrodes lie on the substrate on either a 3/8.lambda. or 5/8.lambda. grid such that adjacent electrodes have either a center-to-center spacing of 3/8.lambda. or a center-to-center spacing of 5/8.lambda.. The electrodes do not have to be physically located at every 3/8.lambda. or 5/8.lambda. grid. The transducer can be withdrawal weighted in a well-known manner in the art to achieve a desired transduction characteristic by actually removing various electrodes or by changing the polarity of certain electrodes. The important element of the invention is to have a 3/8.lambda. or 5/8.lambda. group type sampling which means that however the transducer is withdrawal weighted, the remaining electrodes are always centered on the 3/8.lambda. or 5/8.lambda. grid with any adjacent electrodes having either a 3/8.lambda. or 5/8.lambda. center-to-center spacing depending upon whether the transducer is a 3/8.lambda. structure or a 5/8.lambda. structure. For discussion purposes herein and for ease of explanation, the transducers will be shown and discussed with an electrode centered at each 3/8.lambda. grid or at each 5/8.lambda. grid.
In the embodiment of the novel GSPUDT transducer having 3/8.lambda. group sampling, the physical structure includes a plurality of first pairs of interdigitated electrodes placed on a substrate with each first pair of electrodes having a center-to-center spacing of 3/8.lambda. and reversed polarity from the preceding first pair. A plurality of second pairs of electrodes, each pair with a center-to-center spacing of 3/8.lambda., are interdigitated with the plurality of the first pairs of electrodes. Both electrodes in each second pair have a polarity opposite the polarity of, and a center-to-center spacing of 3/8.lambda. with, the adjacent electrodes of a first pair. The pattern of the electrodes repeats itself each three wavelengths or each five wavelengths. The structure of the 5/8.lambda. sampled GSPUDT is identical to the 3/8.lambda. structure just disclosed except for the 5/8.lambda. center-to-center spacing.
Mass loading only one electrode of each of the second pair of electrodes and its adjacent first pair electrode obtains unidirectional transmission in one direction in the substrate. Mass loading only the other electrode of each of the second pair of electrodes and its adjacent first pair electrode obtains unidirectional transmission in the other direction in the substrate.
The novel transducers can be used on either NSPUDT crystal orientations or conventional crystal orientations because they can be made to radiate in either direction on either CSPUDT crystal orientations or NSPUDT crystal orientations, thus allowing filters, resonators and the like to be built on either conventional or CSPUDT crystal orientations or NSPUDT crystal orientations.